Film forming method and method of manufacturing thin film transistor

ABSTRACT

Provided is a film forming method to minimize decreases in the electrical resistance of an oxide semiconductor film even when a fluorinated silicon nitride film is formed directly on the oxide semiconductor film. The film forming method includes: a surface treatment process in which a substance including an oxide semiconductor film on a substrate is prepared, plasma is generated using a mixed gas of oxygen and hydrogen which contains hydrogen at a rate of 8% or less (not including 0), and plasma is used to treat surface of oxide semiconductor film; a film formation process in which a fluorinated silicon nitride film (a SiN:F film) is subsequently forming on oxide semiconductor film by a plasma CVD method in which plasma is generated using a raw material gas containing silicon tetrafluoride gas and nitrogen gas; and an annealing process in which substrate and film thereon are subsequently heated.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 application of the international PCTapplication serial no. PCT/JP2016/052943, filed on Feb. 1, 2016, whichclaims the priority benefit of Japan application no. 2015-026255, filedon Feb. 13, 2015. The entirety of each of the above-mentioned patentapplications is hereby incorporated by reference herein and made a partof this specification.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of forming a fluorinatedsilicon nitride film (SiN:F film) on an oxide semiconductor film and amethod of manufacturing a thin film transistor using the method.

Description of Related Art

A film forming method in which a fluorinated silicon nitride film (SiN:Ffilm) is directly formed on an oxide semiconductor film (for example, anIGZO film) or is formed thereon with another element interposedtherebetween according to a plasma CVD method using a raw material gascontaining silicon tetrafluoride gas (SiF₄) and nitrogen gas (N₂) isknown (for example, refer to Patent Literature 1).

The fluorinated silicon nitride film has stable electrical insulatingcharacteristics and is denser than a silicon oxide film (SiO₂) that isgenerally used as an insulating film in the related art. Therefore, ithas a characteristic of effectively preventing diffusion of impurities.

CITATION LIST Patent Literature CITATION LIST Patent Literature SUMMARYOF THE INVENTION Problem to be Solved by the Invention

In the film forming method of the related art described above, when afluorinated silicon nitride film is directly formed on an oxidesemiconductor film (that is, without an intervening element such asanother film, the same hereinafter), there is a problem in that anelectric resistance of the oxide semiconductor film is greatly reduced.

This is thought to be caused by the fact that oxygen in the oxidesemiconductor film is reduced by fluorine contained in a raw materialgas when the film is formed, oxygen deficiency occurs in the oxidesemiconductor film, electrons are trapped in holes generated due to theoxygen deficiency, and the trapped electrons serve as donors and greatlyreduce the electric resistance.

When the electric resistance of the oxide semiconductor film is greatlyreduced, for example, a thin film transistor including the oxidesemiconductor film does not exhibit a characteristic as a transistor.

Therefore, the invention provides a film forming method through which itis possible to prevent an electric resistance of an oxide semiconductorfilm from decreasing even if a fluorinated silicon nitride film isdirectly formed on the oxide semiconductor film.

In addition, the invention provides a method of manufacturing a thinfilm transistor using such a film forming method.

Technical Means Solving the Problem

A film forming method according to the invention includes followingsteps. A surface treatment process of preparing a substance in which anoxide semiconductor film is formed on a substrate, generating a plasmausing a mixed gas of oxygen and hydrogen in which a proportion ofhydrogen is 8% or less (not including 0), and treating a surface of theoxide semiconductor film with the plasma is performed. Then, a filmforming process of forming a fluorinated silicon nitride film containingfluorine in a silicon nitride film on the oxide semiconductor film by aplasma CVD method in which a plasma is generated using a raw materialgas containing silicon tetrafluoride gas and nitrogen gas is performed.And then, an annealing process of heating the substrate and the filmthereon is performed.

According to the film forming method, oxygen and hydrogen appropriatelyenter a surface layer portion of the oxide semiconductor film by thesurface treatment process. Further, oxygen and hydrogen that haveentered the surface layer portion of the above oxide semiconductor filmdiffuse in the oxide semiconductor film and restore an electricresistance of the oxide semiconductor film by the annealing process. Asa result, even if the fluorinated silicon nitride film is directlyformed on the oxide semiconductor film, it is possible to prevent anelectric resistance of the oxide semiconductor film from decreasing, andit is possible to maintain the electric resistance in a range in whichthe oxide semiconductor film has semiconductor characteristics.

In the surface treatment process and the film forming process, theplasma may be generated using an inductive coupling type plasmageneration method in which a plasma is generated according to inductivecoupling.

The fluorinated silicon nitride film may be formed as a gate insulatingfilm or a protective film of the thin film transistor using the filmforming method.

Effects of the Invention

According to the invention of claim 1, even if the fluorinated siliconnitride film is directly formed on the oxide semiconductor film, it ispossible to prevent an electric resistance of the oxide semiconductorfilm from decreasing, and it is possible to maintain the electricresistance in a range in which the oxide semiconductor film hassemiconductor characteristics.

According to the invention of claim 2, the following additional effectsare obtained. That is, according to the inductive coupling type plasmageneration method, since a large induced electric field can be generatedin the plasma, the high density plasma is generated and a surfacetreatment in the surface treatment process can be efficiently performed.Also, in the film forming process, silicon tetrafluoride gas andnitrogen gas are efficiently discharged and decomposed, and thefluorinated silicon nitride film can be efficiently formed.

According to the invention of claim 3, the following additional effectsare obtained. That is, while the top gate type thin film transistor hasa structure in which the oxide semiconductor film and the gateinsulating film are in contact with each other, when the abovefluorinated silicon nitride film is forming as the gate insulating filmusing the above film forming method, bonding of Si—F in the film becomesstrong, and fluorine atoms do not easily separate or diffuse in theoxide semiconductor film. Therefore, it is possible to obtain the thinfilm transistor having favorable characteristic stability.

Moreover, since the above fluorinated silicon nitride film serving asthe gate insulating film has stable electrical insulatingcharacteristics, it is possible to obtain the thin film transistorhaving favorable characteristic stability in this regard.

According to the invention of claim 4, the following additional effectsare obtained. That is, while the bottom gate type thin film transistorhas a structure in which the oxide semiconductor film and the protectivefilm are in contact with each other, when the above fluorinated siliconnitride film is forming as the protective film using the above filmforming method, bonding of Si—F in the film becomes strong, and fluorineatoms do not easily separate or diffuse in the oxide semiconductor film.Therefore, it is possible to obtain the thin film transistor havingfavorable characteristic stability.

Moreover, since the above fluorinated silicon nitride film serving asthe protective film is dense, and diffusion of water vapor from theatmosphere into the oxide semiconductor film is effectively prevented,it is possible to obtain the thin film transistor having favorablecharacteristic stability in this regard.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process diagram showing a film forming method according toan embodiment of the invention.

FIG. 2 is a schematic cross-sectional view showing an example of aplasma treatment device that performs an inductive coupling type plasmageneration method.

FIG. 3 is a schematic cross-sectional view showing an example of asample in which a fluorinated silicon nitride film is formed on an oxidesemiconductor film on a substrate.

FIG. 4 is a graph showing measurement results in Table 1.

FIG. 5 is a schematic cross-sectional view showing an example of a topgate type thin film transistor.

FIG. 6 is a schematic cross-sectional view showing an example of abottom gate type thin film transistor.

DESCRIPTION OF THE EMBODIMENTS

(1) Film Forming Method

FIG. 1 shows processes of a film forming method according to anembodiment of the invention.

The film forming method includes a surface treatment process 40, a filmforming process 42 thereafter, and an annealing process 44 thereafter.

The surface treatment process 40 is a process in which a substance inwhich an oxide semiconductor film is formed on a substrate is prepared,a plasma containing oxygen and hydrogen is generated using a mixed gas(O₂+H₂) of oxygen (O₂) and hydrogen (H₂) in which a proportion ofhydrogen (H₂/(O₂+H₂)) is 8% or less (not including 0), and a surface ofthe above oxide semiconductor film is treated with the plasma.

The substrate is, for example, a semiconductor substrate, a glasssubstrate, or a resin substrate, and the present invention is notlimited thereto.

The oxide semiconductor film on the substrate is, for example, an IGZO(In—Ga—Zn—O) film, an ITZO (In—Sn—Zn—O) film, an IWZO (In—W—Zn—O) film,an IZO (In—Zn—O) film, or an ITO (In—Sn—O) film, but the presentinvention is not limited thereto. The oxide semiconductor film may bedirectly formed on a surface of the substrate or may be formed thereonwith another film interposed therebetween.

A plasma treatment time is not particularly limited, and is, forexample, 60 seconds or less. In this case, it is possible to improvethroughput due to the short treatment time.

The film forming process 42 is a process in which a fluorinated siliconnitride film (SiN:F film) containing fluorine in the silicon nitridefilm is formed on the above oxide semiconductor film according to aplasma CVD method in which a plasma is generated using a raw materialgas containing silicon tetrafluoride gas (SiF₄) and nitrogen gas (N₂).

For example, in the same device as the device (for example, a plasmatreatment device) that performs the above surface treatment process 40,in the film forming process 42, a gas that is introduced may be switchedfrom the above mixed gas to a raw material gas without extinguishing theplasma, following the above surface treatment process 40. Therefore,since it is possible to save time and effort of generating a plasmaagain after it is extinguished, it is possible to reduce a treatmenttime and improve throughput.

The annealing process 44 is a process in which the above substrate andthe film thereon are heated (that is, annealed).

The annealing process 44 need not be performed in a vacuum atmosphere,but may be performed, for example, in air. In addition, for example, theair is introduced into a vacuum container of the above plasma treatmentdevice and the process may be performed in the vacuum container. Theheating temperature of the substrate and the like in the annealingprocess 44 may be in a range of, for example, 150° C. to 350° C. Theheating time may be, for example, about 30 minutes to 60 minutes.

According to the film forming method, even if the fluorinated siliconnitride film is directly formed on the oxide semiconductor film, it ispossible to prevent an electric resistance of the oxide semiconductorfilm from decreasing and the electric resistance can remain in a rangein which the oxide semiconductor film has semiconductor characteristics.This is thought to be achieved by the following operations.

That is, oxygen and hydrogen appropriately enter a surface layer portionof the oxide semiconductor film by the above surface treatment process.This oxygen compensates for oxygen deficiency in the oxide semiconductorfilm that occurs in the film forming process. The above hydrogeneliminates (terminates) defects in the oxide semiconductor film.Further, oxygen and hydrogen that have entered the surface layer portionof the above oxide semiconductor film diffuse in the oxide semiconductorfilm and restore an electric resistance of the oxide semiconductor filmby the above annealing process. As a result, even if the fluorinatedsilicon nitride film is directly formed on the oxide semiconductor film,it is possible to prevent an electric resistance of the oxidesemiconductor film from decreasing, and it is possible to maintain theelectric resistance in a range in which the oxide semiconductor film hassemiconductor characteristics.

However, when a proportion of hydrogen in the mixed gas in the surfacetreatment process exceeds 8%, the electric resistance of the oxidesemiconductor film is hardly restored even if the annealing process isperformed. This is thought to be caused by the fact that an amount ofhydrogen in the film becomes excessive, the excess hydrogen serves as adonor, and thus an electric resistance is reduced.

The above operations and effects will be further described below withreference to examples.

In the above surface treatment process 40 and film forming process 42,the above plasma may be generated using an inductive coupling typeplasma generation method in which a plasma is generated according toinductive coupling. An example of the plasma treatment device thatperforms an inductive coupling type plasma generation method is shown inFIG. 2.

The plasma treatment device includes a vacuum container 10 which isvacuum-exhausted by a vacuum exhaust system 12 and into which a gas 16is introduced through a gas inlet 14, and a substrate holder 18 forholding a substrate 2 including the above oxide semiconductor film 4 isprovided therein. As shown in this example, a bias voltage (for example,a negative bias voltage) may be applied to the substrate holder 18 froma bias power supply 20.

In this example, above the substrate holder 18 in the vacuum container10, a linear high frequency antenna 24 is arranged along a surface ofthe substrate holder 18. The vicinities of both ends of the highfrequency antenna 24 penetrate through two openings 22 provided on wallsurfaces that face the vacuum container 10, and an insulating member(for example, an insulating flange) 26 is provided in the openings 22.In this example, the high frequency antenna 24 of a portion positionedinside the vacuum container 10 is covered with an insulating cover 28.Note that packing for vacuum sealing (for example, an O-ring) isprovided between the insulating member 26 and the vacuum container 10,and between the high frequency antenna 24 and the insulating member 26,but these are not shown.

A high frequency current I_(R) flows to the high frequency antenna 24from a high frequency power supply 30 through a matching circuit 32. Afrequency of the high frequency current I_(R) is generally, for example,13.56 MHz, but the present invention is not limited thereto.

In the plasma treatment device, when the high frequency current I_(R)flows to the high frequency antenna 24, a high frequency magnetic fieldis generated around the high frequency antenna 24, and an inducedelectric field is generated in a direction opposite to that of the highfrequency current I_(R) accordingly. According to the induced electricfield, electrons are accelerated to ionize the gas 16 in the vicinity ofthe high frequency antenna 24, and a plasma (that is, inductive couplingtype plasma) 34 is generated in the vicinity of the high frequencyantenna 24 inside the vacuum container 10. Therefore, a method ofgenerating the plasma 34 is called an inductive coupling type plasmageneration method. The above plasma 34 diffuses to the vicinity of thesubstrate 2 and a desired treatment can be performed on the oxidesemiconductor film 4 on the substrate 2 by the plasma 34.

That is, when the above-described mixed gas of oxygen and hydrogen isused as the gas 16, the plasma 34 is generated according to theinductive coupling type plasma generation method and the surface of theoxide semiconductor film 4 can be treated with the plasma 34. That is,the above-described surface treatment process 40 can be performed.Moreover, according to the inductive coupling type plasma generationmethod, since a large induced electric field can be generated in theplasma 34, the high density plasma 34 is generated and a surfacetreatment in the surface treatment process 40 can be efficientlyperformed.

In addition, when a raw material gas containing the above-describedsilicon tetrafluoride gas and nitrogen gas is used as the gas 16, theplasma 34 is generated according to the inductive coupling type plasmageneration method, and the above-described fluorinated silicon nitridefilm can be formed on the oxide semiconductor film 4 according to aplasma CVD method using the plasma 34. That is, the above-described filmforming process 42 can be performed. Moreover, although silicontetrafluoride gas and nitrogen gas are less likely to be discharged anddecomposed than silane (SiH₄) and ammonia (NH₃) which are generally usedin the related art, since a large induced electric field can begenerated in the plasma 34 according to the inductive coupling typeplasma generation method, silicon tetrafluoride gas and nitrogen gas canbe efficiently discharged and decomposed in the film forming process 42.As a result, the high density plasma 34 is generated and the fluorinatedsilicon nitride film can be efficiently formed.

Examples

As shown in FIG. 3, a substance in which an IGZO film with a thicknessof 50 nm is formed as the oxide semiconductor film 4 on the glasssubstrate 2 is arranged inside the plasma treatment device as shown inFIG. 2, a mixed gas (a proportion of hydrogen is shown below) of oxygen(O₂) and hydrogen (H₂) is introduced as a gas, a plasma is generatedaccording to the inductive coupling type plasma generation method, and asurface of the IGZO film is exposed to the plasma for a surfacetreatment. That is, the surface treatment process is performed.Treatment conditions in this case are as follows.

Proportion of hydrogen in mixed gas (H₂/(O₂+H₂)): 0%, 5.7%, 9.1% or23.1%

Treatment time: 60 seconds

Vacuum container internal pressure: 4 Pa

Then, in the above plasma treatment device, a gas is switched to a rawmaterial gas (a ratio of both gases is shown below) including silicontetrafluoride gas (SiF₄) and nitrogen gas (N₂), a plasma is generatedaccording to the inductive coupling type plasma generation method, and afluorinated silicon nitride film (SiN:F film) 6 is formed on the IGZOfilm according to a plasma CVD method using the plasma. That is, thefilm forming process is performed. Thus, a sample shown in FIG. 3 isobtained. Film forming conditions in this case are as follows.

Ratio of both gases in raw material gas . . . SiF₄:N₂=1:1

Pressure in vacuum container: 4 Pa

Film thickness of SiN:F film: 100 nm

Then, the above sample is extracted in the air, heated on a hot plate,and annealed. That is, the annealing process is performed. Annealingconditions in this case are as follows.

Heating atmosphere: air

Heating temperature: 350° C.

Heating time: 1 hour

Sheet resistances of the IGZO film before and after annealing accordingto the hydrogen proportions in the above mixed gas are measured. Theresults are shown in Table 1. In addition, FIG. 4 shows a graph ofmeasurement results in Table 1.

TABLE 1 File: 61407usf-true translation Proportion of hydrogen (H₂)Sheet resistance of IGZO film (Ω/

) in mixed gas Before annealing After annealing 23.1% 5.82E+02 7.94E+029.1% 5.84E+02 6.56E+02 5.7% 4.47E+02 6.18E+05 0.0% 6.76E+03 1.21E+08

The initial sheet resistance (that is, before the above treatment isperformed) of the IGZO film is 5 E+6Ω/□. At any hydrogen proportion, thesheet resistance of the IGZO film before annealing is greatly reducedcompared to the initial sheet resistance. This is thought to be causedby the fact that oxygen and/or hydrogen that have entered the surfacelayer portion of the IGZO film in the surface treatment processaccumulates in the surface layer portion in a large amount, and thusserves as donors and greatly reduces the sheet resistance of the IGZOfilm.

On the other hand, the sheet resistance of the IGZO film after annealingsignificantly greatly increased when the hydrogen proportion is 0%. Whenthe hydrogen proportion is 5.7%, the sheet resistance considerablyincreased. When the hydrogen proportion is 9.1% or more, the sheetresistance hardly increased. An increase in the sheet resistance isthought to be caused by the fact that oxygen and hydrogen that haveentered the surface layer portion of the IGZO film diffuse in the IGZOfilm due to annealing and restore the sheet resistance of the IGZO film.When the hydrogen proportion is 9.1% or more, the sheet resistancehardly increased. This is thought to be caused by the fact that anamount of hydrogen in the IGZO film becomes excessive and the excesshydrogen serves as donors and thus reduces the sheet resistance.

A range in which the oxide semiconductor film such as an IGZO filmnormally has semiconductor characteristics in a thin film transistor orthe like is a range in which the sheet resistance is generally 1 E+5Ω/□to 1 E+8Ω/□. When the hydrogen proportion is 0%, the sheet resistance ofthe IGZO film slightly exceeds the above upper limit 1 E+8Ω/□. On theother hand, in FIG. 4, when the hydrogen proportion is 8%, the sheetresistance of the IGZO film is thought to be about the above lower limit1 E+5Ω/□. Therefore, the hydrogen proportion in the mixed gas ispreferably 8% or less (not including 0).

(2) Method of Manufacturing Thin Film Transistor

Next, a method of manufacturing a thin film transistor using the abovefilm forming method will be exemplified.

FIG. 5 shows an example of a top gate type thin film transistor. A thinfilm transistor 50 a has a structure in which an oxide semiconductorfilm 56 is formed on a substrate 52 with a diffusion preventing film 54interposed therebetween, a gate insulating film 60 is directly formedthereon, a gate electrode 62 is formed thereon, and, in this example, aprotective film 64 is additionally formed thereon. The oxidesemiconductor film 56 is, for example, an oxide semiconductor film suchas the IGZO film described above. This is the same as a thin filmtransistor 50 b shown in FIG. 6.

In this example, a source region 57 and a drain region 58 are formed onboth left and right sides of the oxide semiconductor film 56, and asource electrode 66 and a drain electrode 68 are connected thereto,respectively. However, a channel region remains above both left andright sides of the oxide semiconductor film 56 in place of the sourceregion 57 and the drain region 58, and a source electrode and a drainelectrode are arranged in an overlapping manner. This is the same as thethin film transistor 50 b shown in FIG. 6.

While the thin film transistor 50 a has a structure in which the oxidesemiconductor film 56 and the gate insulating film 60 are in contactwith each other, when the above fluorinated silicon nitride film isformed as the gate insulating film 60 using the above film formingmethod, bonding of Si—F in the film 60 becomes strong, and fluorineatoms do not easily separate or diffuse in the oxide semiconductor film56. Therefore, it is possible to obtain the thin film transistor havingfavorable characteristic stability.

Moreover, since the above fluorinated silicon nitride film serving asthe gate insulating film 60 has stable electrical insulatingcharacteristics, it is possible to obtain the thin film transistorhaving favorable characteristic stability in this regard.

FIG. 6 shows an example of a bottom gate type thin film transistor. Thethin film transistor 50 b has a structure in which the gate electrode 62is formed on the substrate 52, the gate insulating film 60 is formedthereon, the oxide semiconductor film 56 is formed thereon, and theprotective film 64 is directly formed thereon. A structure around asource and a drain is the same as in FIG. 5.

While the thin film transistor 50 b has a structure in which the oxidesemiconductor film 56 and the protective film 64 are in contact witheach other, when the above fluorinated silicon nitride film is formed asthe protective film 64 using the above film forming method, bonding ofSi—F in the film 64 becomes strong, and fluorine atoms do not easilyseparate or diffuse in the oxide semiconductor film 56. Therefore, it ispossible to obtain the thin film transistor having favorablecharacteristic stability.

Moreover, since the above fluorinated silicon nitride film serving asthe protective film 64 is dense, and diffusion of water vapor from theatmosphere into the oxide semiconductor film 56 is effectivelyprevented, it is possible to obtain the thin film transistor havingfavorable characteristic stability in this regard.

-   -   2 Substrate    -   4 Oxide semiconductor film    -   6 Fluorinated silicon nitride film    -   16 Gas    -   24 High frequency antenna    -   30 High frequency power supply    -   34 Plasma    -   40 Surface treatment process    -   42 Film forming process    -   44 Annealing process    -   50 a, 50 b Thin film transistor    -   52 Substrate    -   56 Oxide semiconductor film    -   60 Gate insulating film    -   62 Gate electrode

What is claimed is:
 1. A film forming method comprising: performing asurface treatment process of preparing a substance in which an oxidesemiconductor film is formed on a substrate, generating a plasma using amixed gas of oxygen and hydrogen in which a proportion of hydrogen is 8%or less (not including 0), and treating a surface of the oxidesemiconductor film with the plasma; then performing a film formingprocess of forming a fluorinated silicon nitride film containingfluorine in a silicon nitride film on the oxide semiconductor film by aplasma CVD method in which a plasma is generated using a raw materialgas containing silicon tetrafluoride gas and nitrogen gas; and thenperforming an annealing process of heating the substrate and the filmthereon.
 2. The film forming method according to claim 1, wherein in thesurface treatment process and the film forming process, the plasma isgenerated using an inductive coupling type plasma generation method inwhich a plasma is generated by inductive coupling.
 3. A method ofmanufacturing a thin film transistor, comprising: forming an oxidesemiconductor film on a substrate, forming a gate insulating film on theoxide semiconductor film, and forming a gate electrode on gateinsulating film to form a structure of a top gate type thin filmtransistor, wherein a fluorinated silicon nitride film as the gateinsulating film is formed using the film forming method according toclaim
 1. 4. A method of manufacturing a thin film transistor,comprising: forming a gate electrode on a substrate, forming a gateinsulating film on the gate electrode, and forming an oxidesemiconductor film on gate insulating film, and forming a protectivefilm on the oxide semiconductor film to form a structure of a bottomgate type thin film transistor, wherein a fluorinated silicon nitridefilm as the protective film is formed using the film forming methodaccording to claim
 1. 5. A method of manufacturing a thin filmtransistor, comprising: forming an oxide semiconductor film on asubstrate, forming a gate insulating film on the oxide semiconductorfilm, and forming a gate electrode on gate insulating film to form astructure of a top gate type thin film transistor, wherein a fluorinatedsilicon nitride film as the gate insulating film is formed using thefilm forming method according to claim
 2. 6. A method of manufacturing athin film transistor, comprising: forming a gate electrode on asubstrate, forming a gate insulating film on the gate electrode, andforming an oxide semiconductor film on gate insulating film, and forminga protective film on the oxide semiconductor film to form a structure ofa bottom gate type thin film transistor, wherein a fluorinated siliconnitride film as the protective film is formed using the film formingmethod according to claim 2.